Exhaust gas discharged from an internal combustion engine (e.g., diesel engine) or a combustion apparatus contains a large amount of particulate matter (PM) that mainly contains soot (graphite). If PM is directly discharged to the atmosphere, environmental pollution occurs. Therefore, a filter apparatus including a diesel particulate filter (DPF) that collects PM is normally provided in an exhaust gas passage.
For example, a honeycomb filter having a honeycomb structure has been known as the DPF (element) included in such a filter apparatus. The honeycomb filter includes a plurality of cells (gas passages) defined by a partition wall formed of a porous ceramic having a number of pores (i.e., has a honeycomb structure), one open end and the other open end of adjacent cells being alternately plugged. When exhaust gas is introduced into one open end of the cells of the canned honeycomb filter, the exhaust gas passes through the partition wall, and is discharged from the other open end of the adjacent cells. PM is collected and removed by the partition wall when the exhaust gas passes through the honeycomb filter, so that the exhaust gas is purified.
However, such a honeycomb filter tends to show a rapid increase in pressure loss due to coat of PM. JP-A-10-249124, JP-A-2006-685, and JP-A-10-263340 disclose honeycomb filters having a novel structure aimed at reducing a pressure loss. These honeycomb filters are characterized in that a honeycomb structure (i.e., porous body) is used as a base (support), and a collection layer that collects PM is formed on the surface of the base. JP-A-10-249124, JP-A-2006-685, and JP-A-10-263340 also disclose a method of producing such a honeycomb filter.